The trimming of electrical structures, such as filters, resonators, and passive components, is required in order to get the system to work in the designed way. This is due to the fact that manufacturing processes may lack the required accuracy that is needed to gain the nominal values used in the design phase. Also, statistical variation within the manufacturing process of printed circuit boards, discrete components, or integrated circuits may create the need for trimming some critical parts or points. In this content trimming means cutting a structure or a component, for example by laser, until the desired electrical characteristics are achieved.
The problem so far, has been that trimming has been possible only if the component or the structure has been located on the surface of the whole structure. In other words, trimming has been possible on the surface layer. The apparatus used for trimming, such as lasers, can not penetrate through printed circuit boards. Due to this all tolerance critical parts have been placed on the top layer, which causes large circuit modules that are expensive. Structures on the surface must also be covered with a protection layer, such as an over-glaze material. This is an additional process step, increasing the cost of the structure.
Furthermore, conductor materials used on top layers must be resistive to electro-migration. (Surface layers are sensitive to electro-migration, which causes changes in electrical behavior.) For example, in multilayer ceramics technologies Ag—Pd paste material has to be used on a top layer instead of pure Ag paste for preventing unwanted migration effects. Using Ag—Pd paste instead of Ag paste results in higher conductor losses. That degrades, for example, the Q values of resonators and increases insertion loss in filter structures.
The objective of the invention is to eliminate the above-mentioned drawbacks of known solutions. This is achieved in a way described in the claims.